The present invention relates to positioning devices and, more particularly, to linear positioning devices capable of positioning a workpiece in at least first and second angularly displaced axes.
Linear motors conventionally are used in applications requiring the application of modest force along a linear axis. One application, disclosed in U.S. Pat. No. 4,595,870, for example, includes driving one axis of a positioning table for displacing a workpiece along the axis. Such applications are characterized by high precision in positioning, and modest force and acceleration requirements. The linear motor eliminates the windup characteristic of ball-screw drives and substantially reduces the mass of the moving element. Both of these characteristics of linear motors provide benefits in workpiece positioning precision and acceleration.
The force of which linear motors are capable is limited by resistive heating in the windings of the armature of the linear motor. In my U.S. patent application Ser. No. 859,915 now abandoned, I disclose several techniques for cooling a linear motor. In cooling techniques using liquid coolant, I have discovered that it is possible to attain high forces in permanent magnet DC linear motors using a moderate flow of a liquid coolant in thermal contact with the armature windings. Such high force permits such linear motors to be used in applications not previously considered for such a class of motors.
In my U.S. patent application Ser. No. 15,680, I disclose a high-force linear motor adapted for integration into the bed of a machine tool. Certain machine-tool applications require very long travel at high speeds and accelerations. The present invention addresses one such application wherein, in addition to the foregoing requirements, long linear axes must be accommodated.
In my U.S. Pat. No. 4,505,464, I disclose a positioning table which takes advantage of the magnetic attraction between permanent magnets and armature iron for preloading bearing supporting the movable element of the positioning table. Two sets of magnets in orthogonal planes provide bi-directional preloading.
In the above-referenced related patent application, a pair of spaced-apart parallel rails define an X axis along which it is desired to displace a load. The load may include a Y-axis table capable of displacement at right angles to the X axis. A permanent magnet DC linear motor on one of the rails employs a plurality of permanent magnets affixed to the rail and an armature movable by control signals along the rail. An outboard linear motor assembly is supported on the second rail. The Y-axis assembly is supported between the linear motor and the outboard assembly. An angle of magnetic attraction between the armature and the permanent magnets is controlled to direct the static magnetic attractive force in a direction which, when combined with static and dynamic loads, produces a resultant force in a desired direction. The related application also discloses embodiments with rectangular rails, resin inserts in the rails for more easily attaining a precise surface finish, composite rails with a magnetic metal strip backing the permanent magnets, an integrated motor saddle and air bearing assembly, and damping material in the rails for damping vibrations. The disclosure of this related patent application is incorporated herein for background material.
It is believed that there may be apparatus having first and second parallel rails with a first linear motor on the first rail driving one end of the Y-axis assembly, and a second linear motor on the second rail driving the other end of the Y-axis assembly. It is believed that this apparatus controls each of the first and second linear motors in a conventional manner. That is, a conventional motor controller with velocity and/or position feedback devices is provided for each linear motor. Motor controllers and feedback devices are expensive and providing two of them adds a significant cost burden to the resulting system.